Toxic Lake FAQ

↑ Aerial photo of Harbor Isle Lake | Feb 17, 2020

Illustrates bright green toxic scum accumulation areas with south-west winds, and overall bright green tinted water.

Image Courtesy of: city resident

What is cyanobacteria?

Cyanobacteria are a group of microscopic organisms officially classified as bacteria, but they display characteristics of algae and bacteria. They are found naturally in all types of water. Cyanobacteria perform photosynthesis. When conditions become favorable, however, these organisms can multiply quickly and become dominant in the water body. The color of the chlorophyll required for their life cycle process produces the green coloration and the commonly used name of “blue-green algae".

Cyanobacteria, however, can be present in the water body without visible signs. Cyanobacteria can control their buoyancy, so they are able to float up and down through the water column, dependent on what is optimal for their growth cycle.

Different species of cyanobacteria can produce different toxins called cyanotoxins. Therefore, it is important to exercise caution, especially when visual and odor characteristics appear.

β-Methylamino- l-alanine (BMAA), an amino acid, is a neurotoxin produced by cyanobacteria. BMAA is currently the subject of ongoing scientific research and being studied for its potential role in various neurodegenerative diseases (e.g. ALS, Alzheimer's, Parkinson's).

What type of cyanobacteria does Harbor Isle Lake have?

The FL-DEP analyzed the dominant cyanobacteria in Harbor Isle Lake to be: Microcystis aeruginosa

The lake contains Microcystin toxin and as of Feb 19 2020, also Cylindrospermopsin toxin.

In Harbor Isle lake,

For 2021: the highest measured measured Microcystin concentration was: 610 micrograms/L (or ppb)

This was from the sample at the Southern Entrance Lobe, on Jan 19 2021.

For 2020: the highest measured Microcystin concentration was: 560 micrograms/L (or ppb)

Cylindrospermopsin at 2.7 micrograms/L (or ppb) was also detected.

This was from the sample at the Southern Entrance Lobe, on Feb 19 2020.

More details on recent sampling can be found by clicking the page button, below:

How can the toxin concentrations and visible amounts of blue-green algae be so different in areas of the lake?

The most reasonable answer to this question is through wind fetch. Winds and their direction cause the suspended cyanobacteria in the water to be pushed into certain areas of the lake, where the cells begin to build up against the shore.

When these cell quantities increase, scum mats can occur. These mats are thicker solid-like masses of the cyanobacteria (similar looking to guacamole) near the surface of the water. They usually are bright green in appearance. Due to the cyanobacterial cells’ pigments, they can also have various shades of color from reddish brown-green to bright blue-green or white.

The cyanobacteria cells’ ability for buoyancy regulation, as previously mentioned, can affect how surface aggregations or mats of the blue-green algae can appear suddenly and rapidly shift after a relatively short time period, like a few hours.

Some scums may become very dense, with cells frequently concentrated by a factor of 1,000 or more, eventually reaching in some cases, one million-fold concentrations. This highly dynamic variability of cyanotoxin occurrence must be taken into account when monitoring. These scum mats can persist for days or more.

Refer to the diagram below:

What are the safety standards or guidelines for cyanobacteria / cyanotoxin in recreational water?

The EPA has released new recommendations in May 2019 stating that the safe limit for recreational water contact containing cyanotoxins is 8 ppb for Microcystins and 15 ppb for Cylindrospermopsin.

According to the World Health Organization (WHO), concentrations of Microcystin over 10 micrograms/L (or ppb) has a moderate level of relative probability of acute health effects. Concentrations over 20 micrograms/L (or ppb) is considered a high level. Refer to the chart below:

↑ Photo taken Feb 10 2020, Southern (Entrance) Lobe, demonstrating the trapped toxic scum and the growing amount of toxic blue-green algae residue on cattail reeds that can become airborne via wind.

Harbor Isle Toxic Odor Reports

Below is a map that highlights where noxious odors have been reported by multiple citizens. Take note specifically of the public sidewalks and streets of Harbor Isle neighborhood (approx. 80 to 400 ft away from the lake's shore).

Odor Map Last Updated: Feb 25 2020